Adding the additional buttons will allow you to play games not accessible with only the two standard buttons
Step 1.
Remove the Cartridge Guard (Keep the screws)
Step 2.
Remove the battery springs and wires
Step 3.
If using two batteries I would advise making a hole in the side of the left battery bay to route the battery extension cable
Step 4.
Recent replica shells have an additional speaker grill not present on the original. In order to fit the button bracket, place the bracket in place and mark the speaker plastic that needs removing and snip the plastic until the bracket can be laid flat in place.
Step 5.
Use the drill guide to position a pilot hole
Step 6.
Using a stepped 12mm drill bit, slowly enlarge the hole to 12mm from inside of the shell
Step 7.
Use a sharp craft knife to remove any excess plastic around the button hole for a smooth finish